>>2644052get an ultralight tent first. get the bivy if you think you can make do with even less.
they both weigh about the same, there's no weight savings, and the bivy will keep you less dry. the main advantages the bivy has is more configuration options where you can custom select your pitch and airflow to your location, with a bunch of learning curve to go with it. and the fact that you can use the bivy without the tarp on good weather forecast summer days and shit like that. its worse than a ul tent but more versatile & not beginner friendly.
>>2644058you put a tarp over them unless its like a full on waterproof alpine style bivy or a full on waterproof structured bivy tent.
there's two main categories of bivvies. structured/hooped bivvies like
>>2643974 this pic and unstructured ones like the borah anons are recommending (picrel)
they're totally different things, different purposes, both are cool but they don't really do the same thing.
the bivy's usually just to keep bugs out (like the mesh style we told you to get) or splashes out (like gore tex style unstructured ones)
with the bug bivy you'd typically have a ground sheet, the bug bivy, a sleeping bag or quilt, a tarp and hardware to pitch it, all separate and you just pack what you need.